Question

let U be the set of all triangles in a plane. if A is the set of all triangles with at least one angle different from 60 degree, what is A' ?

Answer 1

U=set of all triangles in a plane.

A=set of all triangles with at least one angle different from the 60°
therefore
A'=set of all triangles with no angle different
from 60°
=set of all triangles with all three angles 60(as in an equilateral triangle all angles are 60°)
=set of all equilateral triangles.
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Answer 2

Hope u like my process
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=> U is the set of all triangles in a plane.

=> A is set of all triangles with at least 1 angle different from 60°
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Note :-
=-=-=-=-=
A' = reciprocal of event A
i.e.
If A is event maintaining some conditions.
Then, A' be the one who don't accept those conditions.
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Now,
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A = event of triangles having one of the angle different from 60

So

A' = event of triangles having all angles to be 60°

Now,
If all angles are 60°, then the triangle is said to be equilateral.

So,

A' is the set of all equilateral triangles in set U.

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